The present invention relates to data error screening and, in particular embodiments, to screening for errors in systems in which an error mitigation signal may be produced.
In a typical data transmission system, a stream of data bits is encoded in a transmitter and transmitted over a communications channel that exhibits noise, distortion or other impairments. In the receiver, a corrupted version of what was transmitted is operated on by a decoder to generate an output sequence of bits that is determined to be that which most likely corresponds to the stream of bits that was encoded in the transmitter.
In some such systems, the decoded output sequence undergoes some type of "analysis" resulting either in an acceptance of the decoded output sequence as being correct or in an indication of a potential decoding error and hence a rejection of the decoded output sequence. A typical such analysis involves error detection and/or correction using codes well known in the art, such as parity check information that was included in the encoded stream of data bits. If the decoded output sequence corresponding to the transmitted encoded data proves to have the correct parity, it is released for further processing. Otherwise an error flag is released, responsive to which the system may either (a) request a re-transmission of the encoded data or (b) initiate error mitigation routines. In audio data transmission systems, for example, the error mitigation routines may substitute a so-called error mitigation signal for the audio signal corresponding to the decoded sequence. This approach is analogous to a long-standing practice in video transmission systems. See, for example, The Digital Signal Processing Handbook, CRC Press, 1998, ch. 56.
The decoder may be capable of identifying more than one output sequence corresponding to the encoded stream of data bits. An example of such a decoder is a so-called List Viterbi decoder which utilizes a List Viterbi algorithm to identify the L best candidate decodings corresponding to the encoded stream of bits. The aforementioned analysis, then, may involve the determination of which, if any, decoded output sequence should be selected for further processing.
For certain transmission systems, such as high quality audio systems, the analysis step is particularly important. Errors in a decoded sequence that would be noticeable to a listener may under certain circumstances- for example, if the errors are few in number and isolated-be mitigated upon being detected. On the other hand, errors in a decoded sequence that are undetected during the analysis step and thus are present in the decoded sequence as it is being further processed by an audio decoder-may be very significant, resulting, for example, in a listener hearing clicks during a pause in a musical selection. In such systems, then, it is desirable to be able to further screen decoded sequences for errors which were undetected in the analysis step.